Inductively coupled plasma-mass spectroscopy (ICP-MS) is currently used in at least the electronics, environmental, geological, and medical fields. The plasma used in ICP-MS is generated by ionizing argon gas. Typically, argon having a purity of 99.9% or higher is used in ICP-MS. Sigma Aldrich sells argon at purities of at least 99.998% and at least 99.995%. As a result, these products are capable of containing up to 20 and up to 50 ppm impurities respectively. These impurities may include, but are not limited to, krypton, xenon, carbon dioxide, nitrogen, oxygen, etc.
Interference from small amounts of krypton and xenon in the argon gas may result in inaccuracy of certain elemental analysis, such as strontium, selenium, and rubidium. To provide more accurate results, higher purity argon is required. However, to produce higher purity argon and to remove the krypton and xenon impurities, air separation units must be operated in a non-optimized way, thereby dramatically increasing costs. At this time, it does not appear that the other impurities contained in the argon gas affect the accuracy of the ICP-MS test results.
Metal organic frameworks (MOFs) are porous materials formed by bonds between metal centers and organic multidentate ligands. The porous materials provide large surface area and uniform pores having small dimensions. The pore sizes may be tailored by selection of the metals and ligands. Kuppler et al., Potential Applications of Metal-Organic Frameworks, Coordination Chemistry Reviews (2008). The potential uses of MOFs include, but are not limited to, hydrogen storage, gas separation and purification, catalysts, and sensors. However, the materials are more flexible than classical adsorbents, such as activated carbon and molecular sieves, resulting in lower thermal and chemical stability. Fletcher et al., Flexibility in Metal-Organic Framework Materials: Impact on Sorption Properties, Journal of Solid State Chemistry, 178 (2005) 2491-2510. Currently, thousands of MOFs have been discovered, but only a couple handfuls are commercially available: iron 1,3,5-benzenetricarboxylate, 2-methylimidazole zinc salt, aluminum terephthalate, and copper benzene-1,3,5-tricarboxylate.
Iron 1,3,5-benzenetricarboxylate (Fe-BTC or Fe3(BTC)2) is commercially available as Basolite™ F 300 from BASF. Fe-BTC is green and has a bulk density of 0.16-0.35 g/cm3, BET surface area of 1300-1600 m2/g, and pore sizes of 0.55 nm and 0.86 nm.
2-methylimidazole zinc salt (ZIF 8 or MOF-1 or MOF-5) is commercially available as Basolite™ Z 1200 from BASF. ZIF 8 is light blue and has a bulk density of 0.35 g/cm3, BET surface area of 1300-1800 m2/g, and pore size of 0.6 nm.
Aluminum terephthalate (MIL 53) is commercially available as Basolite™ A 100 from BASF. MIL 53 is grey and has a bulk density of 0.4 g/cm3, BET surface area of 1100-1500 m2/g, and pore size of 0.85 nm.
Copper benzene-1,3,5-tricarboxylate (Cu-BTC or Cu3 (BTC)2 or HKUST-1) is a widely studied MOF and is commercially available as Basolite™ C 300 from BASF. Cu-BTC is blue and has a bulk density of 0.35 g/cm3, BET surface area of 1500-2100 m2/g, and pore sizes of 0.5 nm and 0.9 nm. Cu-BTC also has a particle size distribution 15.96 um (D50). The pore network of Cu-BTC has a simple cubic symmetry and two kinds of pores with smaller sizes. The crystal structure of Cu-BTC is reported to be composed of Cue (COO)4 paddle wheels with copper dimers as four connectors and benzene-1,3,5-tricarboxylate (BTC) as three connectors, forming a three-dimensional network with main channels of a square cross-section of ca. 0.9 nm diameter and tetrahedral side pockets of ca. 0.5 nm, which are connected to the main channels by triangular windows of ca. 0.35 nm diameter, as shown in FIG. 1. These characteristics have attracted much attention for gas separation, adsorption, sensors, and catalysts.
Sandia National Laboratories issued a report in October, 2008, entitled Computational Investigation of Noble Gas Adsorption and Separation by Nanoporous Materials. Based on molecular simulations, the report indicates that a copper-based MOF, Cu-BTC, selectively adsorbs xenon and krypton atoms when present in trace amounts in atmospheric air samples (79.7 nitrogen/20.0% oxygen).
While MOFs have been proposed for a variety of uses, there still exists a need to be able to separate small quantities of krypton and xenon from argon.